Positioning and Measure
At some point, it is likely that geospatial fieldwork you do will involve some form of measurement related to position, elevation, perimeter, or area. When taking field measurements, it is useful to have a highly accurate starting point, or control point, as a reference. A control point is a previously surveyed position, such as a USGS survey mark. A survey mark is a small engraved metal disc used to mark vertical and horizontal control points. The National Geodetic Survey (NGS) maintains the National Spatial Reference System (NSRS), which has records on hundreds of thousands of control points across the United States. Information about local control points is found on their website using the link above.
Watch Geospatial Infrastructure: Informing Adaptation to Sea Level Rise by the COMET Program/MetEd to learn more.
Watch Precision and Accuracy in Geodetic Surveying by the COMET Program/MetEd to learn more.
Theodolite and Total Station
There are several common tools used for positioning and measure. Earlier in this chapter, I discussed the methods of determining position using a compass. While it is possible to use a compass to delineate a boundary or establish a position, other tools are more suited to this task. A transit is a land-surveying instrument that measures vertical and horizontal angles using a tripod, telescope, and bubble level. A theodolite is a modern version of the transit that includes electronic components.
A more expensive, but highly useful piece of equipment, is called a total station. The total station works like a theodolite, but also includes electronic distance measurement (EDM) capabilities. EDM devices emit electromagnetic waves at the target and measure the distance using the return signal.
Watch Theodolite 1 – Intro & Setup by OTENBuildingCourses to learn more.
A simple way of measuring out from a control point is called a traverse. A traverse starts at a known position, such as a control point, and measures angles and distance to another point. The traverse is completed when the angle and distance to each additional point are measured. A closed traverse ends at the starting point. Any other traverse that does not stop at the starting point is an open traverse. Once you calculate the azimuths in a traverse, you can use the angles and distance from the control point or origin, to calculate grid coordinates (x,y), by using the following equation:
Watch Working with Azimuths Part 1 and Part 2 by Suorsafam to learn more.
Triangulation versus Trilateration
Another common survey method used to extend horizontal control points is triangulation. Triangulation is a method of calculating distances using the geometry of triangles, where the length and angle of one or more sides are known. In this case, the known length called a base, would be the distance between the control point and a second point. Distance to a third or any number of additional points can be determined using the length of the base and the angle from the base to the third point. Any number of other triangles can be calculated using the trigonometric law of sines. Trilateration is another survey method that uses the geometry of triangles. This method applies only measured distances from point to point and the trigonometric law of cosines to find interior angles.
Watch What is triangulation? By Dr. Chris Tisdell to learn more.